Modern vehicles are often provided with performance monitoring devices that use different types of sensors to record various characteristics related to the vehicle's motion and the environment. Known sensors include speed sensors, acceleration sensors, power sensors, temperature sensors, and air flow sensors. Although originally such sensors were mostly used in complex self-propelled vehicles such as airplanes and cars, their use has lately expanded to human-powered devices such as bicycles. Modern high-performance bicycles are often equipped with performance monitoring devices that enable measuring pedal torque, power output and energy expended by direct or indirect measurements of the forces that the rider is applying to drive a vehicle forward. Examples include mechanical strain gauges installed in crank mechanisms or wheel hubs to measure the applied forces and speeds, and thus derived indications corresponding to power and energy.
U.S. Pat. Nos. 7,377,180 and 7,387,029 assigned to Velocomp, LLP, which are incorporated herein by reference and referred to hereafter as '180 and '029 respectively, disclose an apparatus for measuring total force in opposition to a moving vehicle, such as a bicycle, and method of using thereof, which includes a front-facing sensor for measuring static and dynamic pressure, sensors of speed and acceleration of the bicycle, and a microprocessor that receives data from the sensors and calculates power expended by the rider or other power source by finding the total of all forces impinging upon the vehicle and ride. The apparatus further requires an acceleration sensor to measure the acceleration in the direction of travel.
The differential pressure sensor disclosed in '180 and '029 is a front-facing Pitot tube, which provides information on the aerodynamic pressure Q against the front of the vehicle; this information is then used to calculate the opposing aerodynamic force Fair based on an assumption that this force is equal to Q times an aerodynamic factor that defined in '180 as the product of a drug coefficient times frontal area of the vehicle. This product is also referred to in the art as the ‘drag area’, ‘CdA’ or ‘CxA’, and is an important parameter defining the strength of the aerodynamic drag for a vehicle.
In the method described in the '180 and '029 patents, the aerodynamic factor is estimated using a “coast-down calibration” procedure, which involves the vehicle gaining a certain (high) speed, then stopping all pedaling or power input and letting the vehicle coast down to a predetermined (low) speed while the rider maintains his usual riding position. During the coast-down period the system records sensors readings, which are then used by a curve fitting technique to determine static (rolling friction) and dynamic (wind) forces, including an averaged value of the aerodynamic factor (drag area) of the vehicle.
An alternative method of determining a rolling resistance coefficient Crr and the drag area, or CdA, for a vehicle has been disclosed by H. W. Schreuder (Dec 2002) and Robert Chung (April 2003) and is referred to as the virtual elevation (VE) method. This method does not require an acceleration sensor, and includes having the vehicle equipped with a speed sensor and, optionally, a power sensor travel along a route having known elevation features, and then post-processing collected trip data to compute various ‘virtual elevation’ (VE) profiles for the route using different trial values for the Crr and CdA parameters. The correct the Crr and CdA values results in a VE profile that matches known elevation features of the route.
One disadvantage of the conventional VE method and the method of '180 is that these prior art methods can only determine an average value of the effective drag area, or the CdA parameter, during the trip. In reality, the effective drag area may depend on the direction of the airflow with respect to the direction of the vehicle's motion, and thus changes in dependence on the strength and direction of wind and/or vehicle motion direction with respect to the wind. The dependence of the aerodynamic drag area on the airflow angle, which is an important aerodynamic characteristic of the vehicle, is not attainable by the aforedescribed prior art methods. Conventionally, this dependence is determined in a wind tunnel, which is expensive and may not be easily available, especially for a typical, or even a professional, bicycle rider.
An object of the present invention is to provide a method, system and apparatus that may be used in a vehicle to determine the dependence of the aerodynamic drag area thereof on the airflow direction.